The following description is provided to assist the understanding of the reader. None of the information provided or references cited is admitted to be prior art to the present invention.
Staphylococcus aureus (S. aureus) is a cause of a variety of conditions in humans, including skin infections (e.g. folliculitis, styes, cellulitis, impetigo, and furuneulosis), pneumonia, mastitis, phlebitis, meningitis, scalded skin syndrome, osteomyelitis, urinary tract infections, and food poisoning. S. aureus is also a major cause of hospital-acquired (HA or nosocomial) infection of surgical wounds. Therefore, it is desirable to have a diagnostic assay to detect S. aureus. Additionally, methicillin-resistant S. aureus (MRSA) has emerged and become exceedingly prevalent as a nosocomial pathogen. Therefore, it is also desirable to have a diagnostic assay that distinguishes methicillin-sensitive strains of S. aureus (MSSA) from methicillin-resistant strains.
MRSA is one of the two “most out of control” antibiotic resistant pathogens; vancomycin-resistant enterococcus is the other (Society for Healthcare and Epidemiology, SHEA guidelines 2003). Over 50% of nosocomial infections in intensive care units are due to MRSA (National Nosocomial Infections Surveillance System, NNIS report, January 1992-June 2004). Accordingly, MRSA represents a significant threat to public health.
Hospital acquired (HA) MRSA is typically controlled by monitoring patients and personnel for infection. Contact precautions and/or patient isolation may be appropriate when an infection develops or to prevent infections to individuals particularly at risk. The prevalence of Community acquired (CA) MRSA is also increasing. CA-MRSA is defined as MRSA acquired in persons with no known risk factors for MRSA infection (e.g. recent hospitalization, contact with infected patient). In clinical activities, the quick and reliable identification of MRSA has become important for the diagnosis and treatment of infected patients as well as for implementation and management of hospital infection control procedures.
Methicillin resistance is caused by the acquisition of an exogenous gene mecA that encodes penicillin-binding protein (PBP2a or PBP2′). mecA is carried on a mobile genetic element called Staphylococcal cassette chromosome mec (SCCmec) which also contains the ccr gene complex encoding the recombinases necessary for the element's mobility. The SCCmec cassette is a large element that can move in and out of the S. aureus genome. SCCmec integrates at a specific site (attBscc) near the chromosomal origin of replication of S. aureus within the 3′ end of the orfx gene, which has no known function. There are a variety of different types of SCCmec defined by variability in length (approximately 20-60 kb), gene content and other factors such as ccr gene complex type.
The mecA gene is also present in coagulase-negative Staphylococcus (CNS) strains that are less pathogenic than S. aureus. These strains include S. epidermidis, S. haemolyticus, S. sapropliyticus, S. capitis, S. warneri, S. sciuri and S. caprae. Some of these other strains of Staphylococcus inhabit the same environments as S. aureus such as the anterior nares and the skin. It follows that clinical samples such as nasal swabs or wound swabs could potentially contain a mixture of more than one Staphylococcal species. Therefore, detection of mecA alone is not sufficient to identify MRSA directly from clinical sample. Because identification of MRSA is of greater clinical significance than the other Staphylococcus species due to its increased pathogenicity and toxicity, it is desirable that a diagnostic assay distinguish MRSA from the other staphylococcal strains containing the mecA gene.